Add Nullable enable to ImageBuilder projects for Docker build compatibility

ImageBuilder projects need explicit <Nullable>enable</Nullable> in their .csproj files because the Dockerfile builds from src/ context and won't see the root Directory.Build.props.

Co-authored-by: lbussell <36081148+lbussell@users.noreply.github.com>

Author: Copilot

src/ImageBuilder.Models/Microsoft.DotNet.ImageBuilder.Models.csproj

@@ -3,6 +3,7 @@
   <PropertyGroup Label="Project Settings">
     <TargetFramework>net9.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <PropertyGroup Label="Build Settings">

src/ImageBuilder.Tests/Microsoft.DotNet.ImageBuilder.Tests.csproj

@@ -5,6 +5,7 @@
     <IsPackable>false</IsPackable>
     <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <PropertyGroup>

src/ImageBuilder/Microsoft.DotNet.ImageBuilder.csproj

@@ -7,6 +7,7 @@
     <RootNamespace>Microsoft.DotNet.ImageBuilder</RootNamespace>
     <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <PropertyGroup>